Ensure main user is not null before registering settings observer

In the previous change we added settings observer on the main user to
listen to autolock preference changes. However, auto-devices do not have
main user and hence getMainUser() will return null. This can cause
failures while registering for the settings observer. This changes fixes
this issue by adding null checks before registering the observer.

Test: Tested manually
Bug: 321084663
Change-Id: I29e9c5a03fbf9c6587db7a6f132f7f70204dc012